1. Field of the Invention
The invention concerns the field of reaction injection molded polyurethanes.
2. Description of the Prior Art
Reaction Injection Molding (RIM) is a technique for the rapid mixing and molding of large, fast curing urethane parts. RIM polyurethane parts are used in a variety of exterior body applications on automobiles where their light weight contributes to energy conservation. RIM parts are generally made by rapidly mixing active hydrogen containing materials with polyisocyanate and placing the mixture into a mold where reaction proceeds. These active hydrogen containing materials usually comprise a high molecular weight polyhydric polyether (polyol) and a low molecular weight active hydrogen containing compound (chain extender). After reaction and demolding, the parts may be subjected to an additional curing step which comprises placing the parts in an ambient temperature of about 250.degree. F. or greater.
Usual practice is to place all components except the isocyanate in one vessel (polyol, chain extender, tin catalysts, amine catalysts, silicone surfactants, etc. called the B-component) and the isocyanate in another vessel (called the A-component) prior to reaction. Then the A- and B-components are mixed together in the desired stoichiometric balance in a mold as discussed above.
It has been surprisingly discovered that significant advantages occur when all of the tin catalyst is placed in the A-component prior to reacting the A- and B-components in the mold.